1. Field of the Invention
The present invention relates to a device for puffing granular material, in particular cereals and legumes, having a heating apparatus for preheating the granular material, and having a puffing reactor for puffing the material.
The invention further concerns a method for puffing granular material, in particular cereals and legumes, in which the material is first heated in a heating apparatus, and the heated material is then conveyed to the puffing reactor.
2. Related Prior Art
A device and a method of this kind are described in DE 195 21 243 C1.
"Puffing" is understood to mean a treatment method for a granular material, in particular cereals and legumes, that is steam-treated under applied pressure and, when the pressure is abruptly discontinued, is inflated into looser masses. The products are marketed as, for example, puffed wheat, puffed rice, puffed corn, puffed beans, etc. It is also possible to treat other granular material, for example tobacco, in this fashion.
In the 1930s, the facilities were configured in such a way that the material to be treated was introduced into a horizontal rotating cylinder. First the material in the rotating cylinder was heated with gas flames until the steam pressure had risen to a specific value. Then heating with the gas flames was continued until a pressure of approximately 12 bar existed inside the cylinder. The cylinder was then abruptly opened, so that the material shot out and inflated.
Since this operation, which takes several minutes, is relatively time-consuming, in a development of this technology the preheating operation and the actual puffing operation were performed separately, so that these operations could take place concurrently.
In the case of the document cited initially, the material to be treated is first heated in a heating chamber, in which it is preheated to a preheat temperature of approximately 100.degree. C. From the heating chamber, the material is conveyed into a holding container, where the material rests until it is transferred into the puffing reactor.
A similar device is known from GB-B-2 186 180. In this, the heating chamber is configured as a rotating chamber that is heated from outside with gas flames. The material to be heated is continuously passed through the rotating chamber, then drops out of it into a funnel-shaped holding container from which it is conveyed, via a screw conveyor, to the upper charging end of the puffing reactor, where it is once again temporarily transferred into a hopper.
A further design is known from EP-B-0 061 229, in which the material passes through several sieve-tray-like preheating chambers and is then transferred into hoppers, out of which the preheated material is then transferred into the actual puffing reactor.
A device for conditioning soybean fragments is known from CH 656 775 A5. In a fluidized bed system, the soybean fragments are fluidized using air that is introduced through a diffusion floor fitted with numerous nozzles. Heatable heat exchangers are arranged in the reaction space. The fluidized bed system operates continuously, i.e. soybean fragments are continuously introduced and discharged. Residence time is approximately 4 to 8 minutes.
The placement of heat exchangers in the reaction space interferes with fluidization and is hygienically dubious. The physical configuration of the diffusion floor is complex, and the long residence time and continuous operation are not suitable for combining with a puffing reactor that operates with short cycle times of 30 to 90 seconds.
In the case of DE 195 21 243 C1 cited initially, the puffing reactor is configured so that rapid and uniform heat distribution and heat transfer to the material present in the puffing reactor is accomplished, so that very short puffing cycles, in the range of 30 to 90 seconds, can take place.
This makes considerable demands in terms of the speed and uniformity with which the material is preheated in the upstream heating chamber.
Indirect application of heat to the material in the heating chamber, for example by the fact that the chamber wall of the rotating heating chamber is heated from outside with gas flames and that heat is transferred from the heated wall to the material moving along the inner side, requires a certain amount of time and is associated with high heat losses. Uniform heating of the material is also not always guaranteed, since rotating masses of material mix together in relatively uncontrolled fashion, so it is entirely possible for outer regions, which are in direct contact with the hot heating chamber wall for longer periods, to be more strongly heated than portions of the material located at the core of the rotating mass of material.
Maximum uniformity in the heating of the material is, however, a prerequisite for a uniformly good puffed product, since during the short heating period in the puffing reactor there is insufficient time available to completely equalize temperature differences in the batch of material.
It is therefore the object of the present invention to provide a remedy for this problem, and to improve a device and a method of the kind cited initially in such a way that rapid, uniform, and efficient preheating of the material can be achieved.